It just occurred to me that the changelings didn't even really need the Borg. They had infiltrated Starfleet. They can apparently easily tamper with transporters undetected.
All they had to do was infect everyone with "irumodic syndrome", or cancer DNA, or genetic insanity, or whatever. There was tons of ways they could get revenge on the Fed once they had access to transporters that didn't require the timing and flashiness of Frontier Day, and wouldn't even require being bossed around by the nasty Borg at all who will inevitably turn on the changelings anyway.
All they had to do was infect everyone with "irumodic syndrome", or cancer DNA, or genetic insanity, or whatever. There was tons of ways they could get revenge on the Fed once they had access to transporters that didn't require the timing and flashiness of Frontier Day, and wouldn't even require being bossed around by the nasty Borg at all who will inevitably turn on the changelings anyway.